Genetics of twin birth rate in German Holstein and options for breeding

Twinning in dairy cattle poses substantial risks for both cows and calves, including increased rates of calving difficulties, stillbirths, postpartum complications, and negative effects on calf viability and growth. These challenges lead to substantial economic losses and raise serious concerns for animal health and welfare. Consequently, selecting against twin births in German Holstein cattle could be beneficial. In this study, we analyzed the trait twin birth by estimating variance components and genetic correlations with other traits using population-wide data from German Holstein cattle. Breeding values were calculated using a single-step SNP BLUP model, treating twin births as 2 genetically correlated traits: (1) in first parity, and (2) in second and later parities. Heritability was estimated at 0.008 (± 0.0004) for the first parity and 0.026 (± 0.0008) for later parities. Genetic correlations with milk traits were close to zero, slightly positive with fertility, and slightly negative with longevity. The highest genetic correlation of 0.326 (± 0.0229) was observed for stillbirth rate. Comparison of breeding values with daughter phenotypes revealed substantial variability among bulls, whose genetic potential was expressed in varying twin birth rates among their daughters. It is therefore possible to lower twin birth rate through breeding. The estimation of the phenotypic impact of twin births on milk, longevity, calving ease, stillbirth, and health traits highlights the potential to breed against twin births to improve economics, herd performance and animal welfare.